1. Field of the Invention
This invention relates to an optical communication device, for example, an optical transmitting device (LED module or LD module), an optical receiving device (PD device), an optical transmitting/receiving device (LD/PD module or LED/PD module), a WDM filter and other optical devices. The aim of the present invention is to reduce the temperature dependence of the optical properties of the optical communication devices.
This application claims the priority of Japanese Patent Application No. 2000-238002 filed on Aug. 7, 2000 which is incorporated herein by reference.
2. Description of Related Art
An example of the most prevalent PD modules is shown in FIG. 1. The PD module has intrinsically a three-dimensional structure stored in a metallic package. The metallic package has a metallic disc stem 2 having pins 1 below the bottom. A photodiode (PD) chip 4 is mounted via a submount 3 upon the stem 2. A cylindrical cap 6 having a lens 5 is fixed upon the stem 2 above the PD 4. A cylindrical sleeve 7 having an opening above the cap 6 is welded upon the stem 2 at a bottom end. A ferrule 8 is inserted into the axial opening of the sleeve 7. The ferrule 8 seizes an end of an optical fiber 9. The bottom ends of the ferrule 8 and the fiber 9 are slantingly polished.
An elastic bend-limiter 10 is attached to the top of the sleeve 7. The optical fiber 9 carries optical signal light from another unit or a station. The signal light emitted from the optical fiber 9 propagates in the space, passes the lens 5 and enters the PD chip 4 at right angles. The sleeve 7 is aligned by giving light to the fiber 9, moving the sleeve 7 in two-dimensional directions (x and y directions), measuring the light power by the PD 4 and searching the spot which brings the maximum power to the PD 4, and fixed to the most suitable spot. The positive two-dimensional or one-dimensional alignment is indispensable for such a three-dimensional type module for optimizing the spots of the parts. The ferrule 8 is also aligned in the z-direction by supplying test light to the fiber 9, moving the ferrule 8 in the axial direction (z-direction) and seeking the spot for giving the maximum power to the PD, and fixed at the most suitable spot. The cap 6, the sleeve 7 and the ferrule 8 require the alignment. The PD module shown in FIG. 1 is now the most prevalent module which excels in sensitivity, reliability and life time. A similar metallic packaged LD module is also prevalent in the present optical communication networks. The pervasive modules, however, have weak points of the indispensable alignment, the large size and the high cost. A further progress of the optical communication requires a still more reduction of the sizes and costs of LD modules and PD modules. Recent researches are ardently directed to the planar lightguide circuit (PLC) type optical devices.
{circle around (1)} T. Nishikawa, Y. Inaba, G. Tohmon, T. Uno, Y. Matsui, xe2x80x9cSurface Mounting LD Module on a Silicon Substratexe2x80x9d, PROCEEDINGS OF THE 1997 IEICE GENERAL CONFERENCE, C-3-63, p248 (1997),
{circle around (2)} Jun-ichi Sasaki, Masataka Itoh, Hiroyuki Yamazaki, Masayuki Yamaguchi, xe2x80x9cSi bench for highly efficient optical coupling using passively-aligned spot-size converter integrated laser diodexe2x80x9d, PROCEEDINGS OF THE 1997 IEICE GENERAL CONFERENCE, C-3-65, p250 (1997),
{circle around (3)} A. Hirai, R. Kaku, T. Maezawa, K. Takayama, T. Harada, xe2x80x9cSilicon V-Groove Substrate for Optical Modulesxe2x80x9d, PROCEEDINGS OF THE 1997 IEICE GENERAL CONFERENCE, C-3-66, p251 (1997).
These reports propose some kinds of PLC type LD modules and PD modules. These proposed improvements have not been manufactured yet on the practical scale.
An example of the simplest PLC type PD modules is shown in FIG. 2 (plan view) and FIG. 3 (sectional view). The PD module 11 has a silicon bench 12 with an upper step 13 and a lower step 14. The upper step 13 supports an optical fiber 19 and the lower step 14 sustains a PD (photodiode) 15. The PD 15 is a waveguide type PD having a horizontal waveguide with a horizontal sensing region 22. The Si bench 12 has V-grooves 16 and 17 formed in the axial direction by anisotropic etching.
The fiber 19 is partially held by a ferrule 18. The ferrule 18 and the fiber 19 are fixed upon the V-grooves 16 and 17. Another end of the ferrule can be attached to or detached from another optical device. The end 20 of the optical fiber is vertical to the light axis. Light 21 emanating from the end 20 of the optical fiber 19 enters the front end 23 of the PD 15, propagates in the waveguide sensing region 22 and induces photocurrent in the PD 15. The photocurrent is the receiving signal. In the module, the LD is mounted on the same substrate as the PD. The core of the fiber 19 and the sensing region 22 of the PD 15 lie horizontally on a straight line. The PD module 11 is built without alignment. Exclusion of the alignment reduces the cost of manufacturing the PD module 11. Elimination of the lens decreases the part cost. Thus, the module would be a small sized inexpensive PD module. A similar PLC type LD module can be obtained in a similar manner by replacing the PD by an LD in FIG. 2 and FIG. 3.
The prior art of the figures places the optical parts (the PD 15, the ferrule 18 and the fiber 19) on the Si bench 12 horizontally. The fiber 19 is directly coupled to the PD 15 without lens, which reduces the number of parts and the size of the module.
The example makes use of the optical fiber 19 as a medium for introducing the signal light to the module. The optical fiber can be replaced by a light waveguide. The waveguide (front end incidence) type PD can be also replaced by a top-incidence type PD or a bottom-incidence type PD. The situation is similar to the LD module which emits signal light toward the fiber.
The prior art of PLC type modules make V-grooves by anisotropic etching on the single-crystal silicon bench and prints positioning marks on the silicon bench for determining the position of a PD chip or an LD chip. The V-grooves and the positioning marks enable the manufacturers to mount the PD, the LD and the fiber at the desired positions with accuracy. The mounting of the parts at the spots in accordance with the guidance of the marks or the V-grooves is called xe2x80x9cpassive alignmentxe2x80x9d in contrast to the active alignment of the prior art of FIG. 1. The passive alignment enables the PLC type module of FIG. 2 and FIG. 3 to reduce the cost of assembly, the cost of parts and the cost of packaging.
The prior art of FIG. 2 and FIG. 3 has an apparent drawback of the big refractive index difference among the fiber, the PD and the air gap. The large refractive index difference would induce strong reflection at the boundaries of the air and the PD or the fiber. The reflection would arise the difficult problem of the returning light to the LD as well as the energy loss by the reflection. The reflection loss increases in proportion to the square of the refractive index difference. The LD returning light induces instability of the LD oscillation.
An ordinary remedy for reducing the reflection is to fill the air gap between a fiber end 26 and the PD (or LD) with a transparent resin 24, as shown in FIG. 4 or FIG. 5. The transparent resin 24 prevents the signal light from reflecting at the fiber end 26. Silicone-type resins or acrylate-type resins having a refractive index nearly equal to the fiber refractive index (n=1.4xcx9c1.5) are often chosen as the transparent resin. The reflection is reduced nearly to zero at the fiber end since the transparent resin has a refractive index akin to the fiber. Someone have proposed such contrivances of filling the resin for reducing the reflection at the fiber end.
{circle around (4)} Japanese Patent Laying Open No. 7-181343, (181343/""95) xe2x80x9cOptical waveguide part and manufacture of the samexe2x80x9d, suggested a PD module having a waveguide type PD with a slantingly polished front end and a fiber with an end polished slantingly which is glued with an adhesive to the slanting end of the PD. The adhesive which has a similar refractive index to the fiber preferably.
{circle around (5)} Japanese Patent Laying Open No. 5-88041, (88041/""93) xe2x80x9cOptical connecting circuit of optical fiberxe2x80x9d, proposed an optical connection built by inserting two slantingly polished fibers in a glass tube from both ends, supplying an transparent adhesive to the tube via a hole and gluing the ends of the fibers with each other in the tube. The refractive index of the adhesive is nearly equal to the fiber for reducing the reflection loss.
{circle around (6)} Japanese Patent Laying Open No. 60-176003, (176003/""85), xe2x80x9clight power attenuatorxe2x80x9d, suggested a fiber connection made by inserting two fibers into a tube filling an adhesive which has refractive index similar to the fibers at the middle to the tube. The attenuation of light power is controlled by changing the distance between the fiber ends.
{circle around (7)} Japanese Patent Laying Open No. 4-74483, (74483/""92), xe2x80x9cSemiconductor light emitting-devicexe2x80x9d, proposed an LD module having an LD, an optical fiber and a resin-coated junction between the LD and the optical fiber. The resin has a refraction index akin to the fiber.
Many proposals have been filed for reducing the reflection loss at the interfaces between a fiber and a PD, between a fiber and an LD and between fibers by supplying the interfaces with a resin having a refractive index akin to the fiber. These proposals are basically similar to the device of FIG. 4 and FIG. 5. Then, the prior art is described by referring to FIG. 4 and FIG. 5 having a PD or an LD as a typical example.
No problem would occur when the modules are always used in the circumstance at a stable or constant temperature. The surrounding temperature depends upon the regions, the seasons, and the time in a day. Then, electronic devices or optoelectronic devices are required to be available without malfunction in a wide temperature range between xe2x88x9240xc2x0 C. and +80xc2x0 C.
In addition to the applicability to the wide temperature range, the devices are also required to be resistant without inducing degeneration against the frequent temperature changes. Then, a heat cycle test which heats and cools an object device determined times in the temperature range is carried out for examining the temperature property.
The Inventors found that the heat cycle test brings about a decline of sensitivity of the PD and instability of the oscillation of the LD in the lower temperature region between xe2x88x9240xc2x0 C. and 0xc2x0 C. in the prior device of FIG. 4 and FIG. 5. The prior art from {circle around (4)} to {circle around (7)} were unaware of the malfunctions of the devices induced by the heat cycle test. The Inventors have discovered the malfunctions of the PD sensitivity reduction or the LD instability after the heat cycle test for the first time.
FIG. 6 is a graph of the change of the sensitivity as a function of temperature of Sample A (◯) and Sample B (▴) of InGaAs-PDs after the heat cycle test. The abscissa is the surrounding temperature (xc2x0 C.). The ordinate is sensitivity (A/W) of the PDs. Sample A denoted by ◯ has nearly constant sensitivity higher than 1.0 A/W through the wide temperature range between xe2x88x9240xc2x0 C. and +80xc2x0 C. Sample B denoted by ▴, however, reveals serious degradation at the lower temperature region. The sensitivity of Sample B falls to 0.6 A/W at xe2x88x9240xc2x0 C.
These PDs have been manufactured in the same manner by the same processes. The Inventors found the fact that some reveal good properties but others show bad sensitivity at the lower temperature range. What causes the fall of the sensitivity at the lower part of the required temperature range? The curve of ▴ is reversible. When Sample B is heated above 25xc2x0 C., the sensitivity recovers the high values above 1.0 A/W. Sample B can be applicable to the surroundings warmer than 0xc2x0 C. However, PD module should satisfy the requirement of the workableness down to xe2x88x9240xc2x0 C. Then, Sample B should be rejected as a defective one. What is the reason of inducing the low temperature sensitivity degradation of the PD? Why is the sensitivity fall not irreversible but reversible? What invites the low temperature PD sensitivity fall? The Inventors failed in finding reports discussing the problem. The low temperature sensitivity fall is novel itself as a problem for PDs.
The Inventors have become aware of another problem of LDs at a low temperature. The problem of LDs which is different from the mentioned problem of PDs also occurs at the low temperature region. The Inventors have made an LD module having an LD 27, a fiber 28 and a transparent resin 24 covering the air gap between the LD and the fiber, as shown in FIG. 7. The heat cycle test is applied to the LD module. The LD was damaged by the heat cycle test in many points. The heat cycle brings about various defaults at a lower temperatures to the LD module.
One defect is the reduction of the light emission power in accordance with the fall of the temperature. Another default is an increment of returning light 29 reflected at the fiber end, as shown in FIG. 7. A further problem is an occurrence of the kinks (irregular curves) in the relation between the driving current and the LD emission power, as shown in FIG. 8. A fourth defect is an occurrence of double peaks 30 and 31 in the emission spectrum of the LD as shown in FIG. 9. The emission power decline, the increment of the reflection-returning light, the occurrence of the kinks in the current/power relation and the occurrence of double peaks in the emission spectrum are fatal drawbacks for LD modules? What generates such problems? Why are the drawbacks invited in PLC type LD modules?. None of the prior proposals above-mentioned say anything about the problems. None of the cited reports are aware of the existence of the problems.
The defaults will degrade the value of the LD modules or the PD modules as industrial products. Although the PLC can cut down the cost of LD or PD modules, the PLC type modules are still inapplicable to the actual communication system. There are still many problems to be conquered in the PLC technology.
The Inventors appreciate that it is indispensable to solve such problems in order to put the excellent PLC technology into industrial practice. The Inventors scrutinize the ground of the defaults in the resin-packaged modules induced by the heat cycle test. The Inventors observe the state of the resin of the LD modules and the PD modules after the heat cycle test. The microscope observation revealed a surprising fact. The microscope observation notified the Inventors that the resin was damaged by several kinds of air gaps.
As shown in FIG. 10 and FIG. 11, three kinds of defaults have happened in the resin. The defaults are as follows;
A. resin exfoliation at the interface between the potting resin and the fiber end, resin exfoliation at the interface between the PD end and the resin and the exfoliation between the LD end and the resin,
B. air bubbles occurring in the resin in the light path,
C. cracks happened in the resin.
These phenomena have happened in the resin of the PD modules or the LD modules after the heat cycle test. The phenomena are the grounds of the defaults mentioned above. It was a surprising fact which has no suggestion in any reports.
The three kinds air gaps induce the above-described imperfections. The crack C is a planar air gap induced in an intermediate region of the resin. The bubble B is a ball air gap happened in an intermediate region of the resin. The exfoliation A is an air gap appearing at the boundary between an optical part and a resin. Here, the exfoliation which is a ground of making an air gap is here used for expressing the result. These terms indicate all the air gaps occurring in the resin. The terms of the exfoliation A, the bubbles B and the cracks C are named for distinguishing the air gaps by the shapes or the origins.
The exfoliation A results in an increase of the reflection loss at the device end. In the case of the LD, the exfoliation A induces an increment of the reflection-returning light which instabilizes the LD oscillation.
The cracks C and the bubbles B do not only increase the reflection loss but also induce diverging loss of the signal light like concave lenses. Since the bubbles or the cracks are the air gaps with a refractive index (n=1) lower than the resin, they act as concave lenses in the resin. The LD modules or the PD modules are designed on the assumption that the light beams make their straight way in the transparent resin. However, the cracks, the bubbles and the exfoliation will scatter, reflect, refract and diverge randomly in the LD modules or the PD modules. The random reflection, extra refraction, scattering and diverging the signal light will reduce the sensitivity of the PD, the emission power of the LD and instabilize the oscillation mode of the LD.
What induces the decline of the PD sensitivity, the kinks of the LD current/power relation and the double peaks in the LD emission spectrum is the cracks C, the bubbles B and the exfoliation A generated in the resin of the LD modules or the PD modules.
The Inventors intensively considered the reasons why the air gaps incur such as the exfoliation, the cracks or the bubbles. The potting resin is one of the ultraviolet-hardening resins which are hardened by ultraviolet rays radiation at room temperature or one of the thermally-hardened resins which are hardened by heating at, for example, 100xc2x0 C. or 150xc2x0 C.
Thus, the resin will stably fill the space in the modules without air gap at room temperature or higher than the room temperature.
A rigorous surrounding condition requires workableness at xe2x88x9240xc2x0 C. for the PD or the LD modules. The Inventors became aware that the large temperature difference between xe2x88x9240xc2x0 C. and the room temperature incurs the serious problems.
A fall of the temperature invites shrinkage for almost all resins. The potting resins of the communication devices also shrink when the surrounding temperature lowers. An elasticity-rich resin would be able to alleviate the inner stress by allotting the shrinkage selectively to extra, free parts of the resin. The shrunk parts again dilate when the module is heated. But the whole volume is kept to be constant by the external hard package. The package prohibits the resins from dilating or reducing freely. The heat-cycle does not change the actual volume but varies the inner stress higher or lower in the resin. Cooling reduces the inner stress and heating raises the inner stress in the package-restricted resin. The heat cycle induces a cyclic change of the inner stress in the hard package. However, the change is not fully reversible. Repetitions of expansions and shrinks deprive the resin of the inherent elasticity. The heat cycle test facilitates aging of the resin. The heat-cycle-aged resin alleviates the inner stress at the heated and dilated state by permanent shrinkage. The permanent shrink increases the expansion inner stress at the cooled shrunk state. The loss of elasticity of the resin cannot compensate the excess tensile stress. Then, the excess tensile stress forces the resin at the joints to exfoliate from the fiber end, the PD front or the LD front, when the adhesive force is weak, as shown in FIG. 10 and FIG. 11. When the adhesive force is stronger than the tensile stress, the excess tensile force makes cracks (C) in the intermediate regions of the resin. The exfoliation (A) and the cracks (C) appear at a low temperature. When the module is heated, the exfoliation (A) and the cracks (C) temporarily disappear, because adjacent resin dilates and kills the gaps. But when it is cooled, the exfoliation (A) and the cracks (C) are revived.
Otherwise, air bubbles (B) which have been generated at the resin-hardening time and have been pressurized in the resin dilate and expand in the heat-cycle-aged resin with poor elasticity at a low temperature. The origins of the air bubbles are inherent bubbles contained in the prehardened, fluid resin, newly-born bubbles occurring at the small cavities of solid parts or newly-made bubbles induced by unwettability of the surfaces of solid parts. The air bubbles disappear at a heated state, since the surrounding resin dilates. The air bubbles expand and dilate at a cooled state, since the adjacent resin shrinks. It was revealed by the present Inventor""s searches that the exfoliation (A), the cracks (C) and the bubbles (B) which have been generated in the potting resin raise various problems above-mentioned. The exfoliation (A) between the PD and the resin incur the reflection whenever the light pass the different mediums among the resin, space and the PD, as shown in FIG. 10. The reflection loss is increased by exfoliation. The same things are induced by the exfoliation generated between the fiber and the resin. The increment of the reflection loss is remarkable. The decrease of the intake power reduces the sensitivity of the PD. At a high temperature, the exfoliation (A), the cracks (C) and the bubbles (B) disappear, the reflection loss decreases and the sensitivity of the PD rises. Serious degradation of Sample B, as shown in FIG. 6, now clear the cause.
In the case of the LD, the exfoliation (A), the cracks (C) and the bubbles (B) invite increments of the reflection loss and reflection-returning light. It""s understandable that the increment of the reflection-returning light makes the kinks, as shown in FIG. 8 and it is also reasonable that an occurrence of double peaks in the emission spectrum of the LD is induced by the increment of the reflection-returning light.
Nobody have expected that silicone-type resins or acrylate-type resins constructing the potting resin which are so elastic happens the exfoliation and the cracks in the potting resin. The heat-cycle generates the exfoliation (A), the cracks (C) and the bubbles (B) in the resin. When the optical devices are maintained at high temperatures, the expansion of the resin conceals the exfoliation (A), the cracks (C) and the bubbles (B). But the optical devices are cooled to low temperatures, the resin shrinkage reveals the exfoliation (A), the cracks (C) and the bubbles (B) in the resin. The ground of inviting the defaults above-mentioned turns out to be the exfoliation (A), the cracks (C) and the bubbles (B) in the resin.
The above analysis suggested a remedy of solving the difficulty by pressurizing the resin by some means to the Inventors. Excess pressurizing would suppress the resin from producing the exfoliation (A), the cracks (C) and the bubbles (B) even at a low temperature. The defects of the exfoliation (A), the cracks (C) and the bubbles (B) are made by the tensile stress (positive inner stress), because the sign of the stress is physically defined as positive for tensile and negative for compressive. If negative (compressive) inner stress is given to the resin, the defects would not appear in the resin even at a low temperature. The problem would be conquered by pressurizing the resin by some contrivance at a low temperature.
The new concept of the present invention is to apply compressive forces showing by arrows D, E and F at all times to the resin as shown in FIG. 12 and FIG. 3. The pressurizing forces which should compress the resin are schematically denoted by the arrows D, E and F. The compressing forces should maintain extra pressure sufficient for suppressing the gap, the exfoliation or the cracks from occurring in the potting resin even at xe2x88x9240xc2x0 C. In the highly-pressurized state, a cooling down to xe2x88x9240xc2x0 C. would not generate the exfoliation (A), the cracks (C) and the bubbles (B), which would exclude the phenomenon of the low temperature sensitivity decline from the PDs. The excess pressurizing of the present invention would rescue the LDs from the increase of the returning light, the double peaks of the emission spectrum and the occurrence of the kinks in the current/power relationship.
The problem is now how to apply the extra pressure to the potting resin. How can the pressure be applied to the inner resin? What can allot the pressure to the resin at a low temperature? A decrease of temperature invites a reduction of pressure in any material due to the positive elasticity. It is difficult to apply pressure to the inner resin at a low temperature. It is, however, not entirely impossible. The Inventors thought of such a device which can afford to maintain the compressive stress in the resin at a low temperature and found a solution for the problem.
Namely, the Inventors obtained a new device which can give compressive stress to the potting resin for suppressing the defects (A), (B) and (C) from occurring. The present invention is based upon the discovery of ground of the sensitivity decline at a low temperature and the contrivance of eliminating the ground.
Till now, the skilled have commonly thought that the resin potting or the plastic molding should be done without applying extra pressure to parts in the device. They believed that the extra pressure would break or distort the inner parts. Too strong compressive stress would break the leads or the chips or distort the chips in the device, which incurs troubles. In the silicon semiconductor electric devices, for example, Si-ICs are sometimes enclosed by a silicone-type resin with rich elasticity. The elastic silicone-type resin protects the chips from compressive stress by the excellent elasticity. The elastic silicone resin-coated chips are further molded with a harder epoxy-type resin for protecting the device from external shocks by enhancing the mechanical strength. The inner softer potting resin need not be transparent. The outer part is a hard (epoxy resin) shell and the Si-IC chips are protected by the softer silicone type resin enclosed by the harder shell. The double resin structure consisting of the harder and softer resins is suitable for resin-packaging of the Si electronic ICs. The double resin packaging is prevalent in xe2x80x9celectronicxe2x80x9d devices.
The simple double resin structure is pertinent to the electronic parts or devices which require no consideration for light, since the parts or devices exchange no light among them. The soft resin potting for protecting chips has been already a common technique for electronic devices.
The potting of soft resins has been recently applied to PLC (planar lightwave circuit) type optoelectronic devices (fibers, waveguides, LDs, PDs or LEDs) which pitch and catch light among them. A harder epoxy resin forms an external package protecting the softer resin and the optoelectronic parts. The double structure applied to the optoelectronic devices is common to the prior electronic devices. But the softer potting resin should be transparent, because the devices exchange light via the potting resin. The main purpose of the resin is not to protect the optoelectronic devices from the external forces but to reduce the reflection loss at the interface between the devices and the fibers/waveguides. A soft, transparent resin which has a refractive index close to the fibers/waveguides should be chosen for the potting resin. No sooner had the double resin molding been applied to the PLC optoelectronic devices than the optoelectronic devices were beset with the mentioned problems: the low temperature sensitivity decline of PDs, the returning light increment, the current/power kinks and the multi-peak emission spectrum of LDs. Encountering the unexpected difficulties, all the skilled have been at a loss. There were two problems. One was the unknown ground of incurring the troubles. The other was lack of the remedy for the troubles.
The Inventors attributed the ground of the troubles to the exfoliation (A), the bubbles (B) and the cracks (C) appearing in the potting resin at a low temperature. The Inventors hit upon a new solution of giving extra pressure upon the potting resin at all times for prohibiting the resin from making the exfoliation (A), the bubbles (B) and the cracks (C) at a low temperature.
Pressurizing requires a closed, airtight vessel for determining the capacity of the space containing the resin and the device chips. The plastic mold type devices have outer shells made of a hard resin. e.g., epoxy resin. This invention makes the best use of the hard resin-molded outer shell as an airtight pressure vessel.
What can apply the extra pressure at all times to the potting resin for prohibiting the resin from making the defects at a low temperature? This is a difficult problem. The epoxy resin for building the outer shell which is a thermosetting resin is hardened by heating up to a high temperature in a transferring molding machine. The pressure acting between the outer epoxy resin and the inner potting resin at the temperature is equal to the inner pressure in the metallic mould of the transferring molding machine. Then, the object resins are cooled to room temperature. The inner pressure is decreased but a positive pressure is still making compressive stress (negative stress) in the potting resin at room temperature.
The inner, softer potting resin has a larger thermal expansion coefficient than the outer harder resin, in general. A further fall of temperature gives the inner potting resin the tendency of shrinkage but the outer harder shell forces the inner softer resin to continue to fill the inner space due to the airtightness of the vessel (the space confined by the outer resin). The inner pressure is converted to negative (less than the atmospheric pressure). The negative pressure produces strong tensile stress in the potting resin. The tensile stress induces the exfoliation (A), the bubbles (B) and the cracks (C) in the potting resin. Since the temperature fall from the heated molding to the lowest limit (xe2x88x9240xc2x0 C.) is quite large, the appearance of the exfoliation (A), the bubbles (B) and the cracks (C) is unavoidable in the conventional double resin packaging.
The present invention interposes a pressurizing element with affluent elasticity between the inner potting resin and the outer hard resin. The rich elasticity means low Young""s modulus or low elastic coefficient which is a ratio of an increment of pressure to an increment of volume. The rich elasticity allows the pressurizing element to expand according to the fall of the temperature. The softness of the pressurizing element ensures the resin a positive pressure by expanding itself at low temperature. The pressurizing element is here called a xe2x80x9cpressurizing balloonxe2x80x9d or a xe2x80x9cpressurizing resinxe2x80x9d. The pressurizing element has higher volume/pressure sensitivity than the potting resin. In brief, the pressurizing element is softer than the potting resin. xe2x80x9cSoftnessxe2x80x9d is equivalent to a low elastic coefficient, low Young""s modulus and rich elasticity. xe2x80x9cSoftnessxe2x80x9d is conspicuous attribute of the pressurizing element.
The present invention maintains the positive pressure applying upon the potting resin by packaging the high pressure sensitive pressuring balloon or pressurizing resin under high initial pressure in contact with the potting resin at the outer (epoxy) resin moulding. The initial high pressure is determined to maintain the positive pressure in the package till the lowest limit (e.g., xe2x88x9240xc2x0 C.) of the desired temperature range. Thus, the inner space always presses the pressurizing element to a size smaller than the normal state at the atmospheric pressure within the required temperature range. The pressurizing element gives the potting resin the extra pressure even at the lowest limit of temperature. The pressurizing element enables the potting resin to expel the exfoliation (A), the bubbles (B) and the cracks (C).